Multi-hop wireless backhaul networks (MWBNs) aim to provide high-speed, long-distance, non-line of sight and low-cost wireless access services, which have made up for the deficiencies of difficult wiring, high costs, limited network coverage area, poor topology flexibility and other problems existing in wired backhaul networks. Because of adverse factory environment and difficult wiring, and to improve the factory production efficiency, there is an urgent need for novel industrial applications, such as digital oilfield, wide area interconnection of smart grids, cooperation of industrial robots and the like, to construct backhaul networks based on management-control integration, so as to complete regional coverage, and achieve hybrid transmission of measurement, control, audio, video and other information. For new requirements of industrial applications, MWBNs have obvious advantages.
However, control information about a device often has strict requirements for the QoS of MWBNs, for example, the reliability of control signals is required to be 100%, and the time delay and jitter are required to be ms level, i.e., MWBNs need to meet the requirements of high real-time, high reliability and the like for control information transmission. As a key technology of MWBNs, the time synchronization technology plays a crucial role in solving the problems of information fusion and coprocessing, and guarantee of real-time, reliability and the like of information transmission. However, the local time of nodes is obtained by counting the output pulses of internal crystal oscillators, and synchronization between nodes is achieved by exchanging time information. Therefore, it is difficult for nodes to achieve precise synchronization in the aspect of time under the influence of difference between internal crystal oscillators and the interference of network communication links under the factory environment.
Based on the existing network communication protocol, the IEEE 1588v2 Precision Time Protocol (PTP) solves the problem of time synchronization in the industrial wired Ethernet, and the current synchronization precision can reach 50 ns. However, the key for PTP to achieve high-precision time synchronization is based on a hardware timestamp and symmetrical links. The so-called hardware timestamp refers to a transceiving timestamp for acquiring network messages near the physical layer. Meanwhile, to further counteract the influence of link asymmetry, PTP establishes a concept of peer-to-peer transparent clocks, thereby effectively guaranteeing the symmetry between uplink and downlink, and then improving the time synchronization precision. However, most Wi-Fi chips at present are of a single-chip structure, i.e., the selection of a Wi-Fi node timestamp is only achieved above the MAC layer, and then the asymmetry of links is seriously affected by the forwarding delay differences of MAC layer queuing, PHY layer transmission and others, and the selection mechanism of timestamps. In most existing methods, a statistical principle is used to perform link delay compensation, where messages in the network are added, and the real-time is poor at the same time. Moreover, it is found by experiment that the synchronization precision of directly applying the PTP to a wireless network can reach ms level. Meanwhile, open source Wi-Fi chips are unavailable in the market, and development costs caused by designing MAC layers of Wi-Fi based on an FPGA are too high, so that it is undesirable to design PTP-based wireless network nodes.